1. Field of the Invention
The present invention relates to a communication apparatus employing a multi-carrier transmission method (Digital Wavelet Multi-Carrier transmission method, which is hereinafter referred to as “DWMC transmission method”), which performs data transmission with digital modulation and demodulation processes utilizing real coefficient wavelet filter banks.
2. Description of the Related Art
The transmission method involving digital modulation and demodulation processes utilizing real coefficient wavelet filter banks is a type of multi-carrier modulation method in which a plurality of digital modulated waves are synthesized using real coefficient filter banks to generate a transmission signal. PAM (pulse amplitude modulation) is used as a method for modulating each carrier.
Data transmission according to the DWMC transmission method will be described with reference to FIGS. 15 to 18. FIG. 16 is a waveform diagram showing an example of a wavelet waveform. FIG. 16 is a waveform diagram showing an example of a waveform transmitted according to the DWMC transmission method. FIG. 17 is a spectrum diagram showing an example of a transmission spectrum according to the DWMC transmission method. FIG. 18 illustrates a frame to show an example of a configuration of a frame transmitted according to the DWMC transmission method.
When data are transmitted according to the DWMC transmission method, as shown in FIG. 15, impulse responses of each subcarrier are transmitted in an overlapping relationship with each other in the subcarrier. Each transmission symbol becomes a time waveform that is a combination of impulse responses in each subcarrier, as shown in FIG. 16. FIG. 17 shows an example of an amplitude spectrum. According to the DWMC transmission method, transmission symbols as shown in FIG. 16 in a quantity ranging from several tens to several hundreds are collected to form one frame to be transmitted. FIG. 18 shows an example of a configuration of a DWMC transmission frame. The DWMC transmission frame includes symbols for frame synchronization and symbols for equalization in addition to symbols for transmitting information data.
FIG. 14 is a block diagram of a communication apparatus according to the related art comprised of a transmitter 299 and a receiver 199 employing the DWMC transmission method.
In FIG. 14, reference numeral 110 represents an A/D converter; reference numeral 120 represents an wavelet transformer; reference numeral 130 represents a P/S converter for converting parallel data into serial data; reference numeral 140 represents a judgment unit for judging a received signal; reference numeral 210 represents a symbol mapper for converting bit data into symbol data to perform symbol mapping; reference numeral 220 represents an S/P converter for converting serial data into parallel data; reference numeral 230 represents an inverse wavelet transformer; and reference numeral 240 represents a D/A converter.
An operation of the communication apparatus having such a configuration will now be described.
First, at the transmitter 299, the symbol mapper 210 converts bit data into symbol data, and symbol mapping (PAM modulation) is performed according to each item of the symbol data. The serial-to-parallel converter (S/P converter) 220 supplies each subcarrier with real numbers “di” (i=1 to M, M being a plural number) which are subjected to an inverse discrete wavelet transform on a time axis by the inverse wavelet transformer 230. Thus, sample values having time axis waveforms are generated to generate a series of sample values representing transmitted symbols. The D/A converter 240 converts the series of sample values into a base band analog signal waveform that is continuous in time, the waveform being then transmitted. The number of the sample values on the time axis generated by the inverse discrete wavelet transform is normally 2 to the n-th power (n is a positive integer).
At the receiver 199, the received signal is converted by the A/D converter 110 into a digital base band signal waveform that is then sampled at the same sample rate as that of the transmitter. The series of sample values is subjected to a discrete wavelet transform on a frequency axis by the wavelet transformer 120 and thereafter serial-converted by the parallel-to-serial converter (P/S converter) 130. Finally, the judgment unit 140 calculates the amplitude of each subcarrier to judge the received signal and obtain received data.
Since amplitude distortions and phase distortions can occur during communication because of impedance fluctuations of the transmission path and the influence of multi-path, a capability of treating both of amplitude and phase parameters, i.e., complex information will be convenient. However, the DWMC transmission method according to the related art does not allow distortions to be corrected depending on the condition of transmission paths because it is only capable of treating amplitude information, which results in a problem in that transmission efficiency is significantly reduced (see the following document, for example).
Hitoshi KIYA “Digital Signal Processing Series 14, Multi-Rate Signal Processing”, Shokodo, Oct. 6, 1995, pp. 186-190
As thus described, a communication apparatus employing a transmission method utilizing real coefficient filter banks according to the related art has a problem in that it can treat only amplitude information as transmission data and in that a receiver cannot perform a process for treating complex information.